1. Field of the Invention
The present invention relates to the generation of chirp signals for use in pulse compression sensors such as radars and sonars and, more particularly, to an ultra-linear chirp generating scheme that generates a plurality of coherently interruptible and frequency selectable chirp sub-pulses over a broad bandwidth.
2. Description of the Prior Art
Pulse compression is presently being used in a variety of radar and sonar systems to detect missiles, aircraft, ground vehicles and people, ships, submarines, and faults in various materials like cast metals. A problem with pulse compression is that desirable high compression ratios require long pulses. But applications that require detection of close-range targets must use short pulses. These applications, therefore, cannot take full advantage of high compression chirp waveforms. If a means were available to break a chirp into sub-sections, with the phase, frequency, and frequency-rate precisely controlled at the break points, then more of these applications could be implemented with high compression ratios. Ideally, the ultimate resolution of the compressed pulse is a function of its total bandwidth. But, in practice, as the bandwidth is increased, the chirp linearity becomes the limiting factor. Also, the price sky-rockets while the reliability and maintainability becomes poorer. If a means were available to reliably generate wide bandwidth highly linear chirps with simple hardware, then products could be made with higher performance (both in terms of resolution and in terms of reliability) and with lower cost.
A chirp is defined mathematically as sin (.omega.t+.psi.t.sup.2) where .psi. is the ramp rate in radians per second per second. Common methods for generating chirp signals include; (1) feeding a ramping voltage into a voltage controlled oscillator (VCO) without feedback; (2) using a delay-line/mixer/reference to provide feedback to (1); (3) using a reference and an impulse sampler to provide feedback to (1); (4) adding memory to (2) or (3) such that the error voltages can be stored on a pulse to pulse basis and thus improve the linearity, and; (5) direct playback of a digitally stored chirp signal. These first four techniques have limited linearity, cannot be coherently started and stopped, and are limited in bandwidth by the linear region of VCO operation. Complex circuits with critical/sensitive parts and adjustments result as attested by methods 2, 3, and 4 being used to correct problems. Method 5 is severely limited in bandwidth since a digital-to-analog (D/A) converter must run at about three times the highest desired output frequency in practical systems. Thus, for example, a high speed 200 MHz D/A converter would provide only a 66 MHz chirp, yet would require a very tight filter specification. The filter would have to both reject the digital harmonics and be flat enough through 66 MHz to maintain the linearity of the chirp. Consequently, it is desirable to have a chirp generator that exhibits high linearity, can be coherently started, stopped and restarted at various frequencies, has respectable bandwidth, and can be economically manufactured.